Field of the Invention
This invention relates to an apparatus for supplying a stream of heated, humidified gases to a user for therapeutic purposes. This invention particularly relates to sensors used in the apparatus for controlling the humidity of a gases stream in devices that provide humidified air for: respiratory humidification therapy, high-flow oxygen therapy, CPAP therapy, Bi-PAP therapy, OPAP therapy, etc, or humidification of gases used for insufflation or keyhole surgery.
Description of the Related Art
Devices or systems for providing a humidified gases flow to a patient for therapeutic purposes are well known in the art. Systems for providing therapy of this type (for example respiratory humidification) have a structure where gases are delivered to a humidifier chamber from a gases source. As the gases pass over the hot water, or through the heated, humidified air in the humidifier chamber, they become saturated with water vapour. The heated humidified gases are then delivered to a user or patient downstream from the humidifier chamber, via a gases conduit and a user interface.
The gases delivery system can be a modular system that has been assembled from separate units, with the gases source being an assisted breathing unit or blower unit. That is, the humidifier chamber/heater and the blower unit are separate (modular) items. The modules are in use connected in series via connection conduits to allow gases to pass from the blower unit to the humidifier unit.
Alternatively, the breathing assistance apparatus can be an integrated system, where the blower unit and the humidifier unit are contained within the same housing in use.
In both modular and integrated systems, the gases provided by the blower unit are generally sourced from the surrounding atmosphere.
A third general form of breathing assistance system, which is typically used in hospitals, is one where the breathing assistance system receives at least a portion of the gases which it uses from a central gases source, typically external to the area of use (e.g. a hospital room). A gases conduit or similar is connected between an inlet which is mounted e.g. in the wall of a patients room (or similar). The gases conduit is either connected directly to the humidifier chamber in use, or a step-down control unit or similar can be connected in series between the gases inlet and the humidifier chamber if required. This type of breathing assistance system is generally used where a patient or user may require oxygen therapy, with the oxygen supplied from the central gases source. It is common for the pure oxygen from the gases source to be blended with atmospheric air before delivery to the patient or user, for example by using a venturi located in the step-down control unit. In systems of the type where at least some of the gases are delivered from a central source, there is no need for a separate flow generator or blower—the gases are delivered from the inlet under pressure, with the step down control unit altering the pressure and flow to the required level.
An example of a known, prior art, type of modular system using atmospheric gases only is shown in FIG. 1A.
In typical integrated and modular systems, the atmospheric gases are sucked in or otherwise enter a main ‘blower’ or assisted breathing unit, which provides a gases flow at its outlet. The blower unit and the humidifier unit are mated with or otherwise rigidly connected to the blower unit. For example, the humidifier unit is mated to the blower unit by a slide-on or push connection, which ensures that the humidifier unit is rigidly connected to and held firmly in place on the main blower unit. An example of a system of this type is the Fisher and Paykel Healthcare ‘slide-on’ water chamber system shown and described in U.S. Pat. No. 7,111,624. A variation of this design is a slide-on or clip-on design where the chamber is enclosed inside a portion of the integrated unit in use. An example of this type of design is described in WO 2004/112873.
One of the problems that has been encountered with systems that provide a flow of heated, humidified gases to a patient via a gases conduit and an interface is that of adequately controlling the characteristics of the gas. Clearly, it is desirable to deliver the gas to the patient (i.e. as it exits the user interface) with the gas at precisely the right temperature, humidity, flow, and oxygen fraction (if the patient is undergoing oxygen therapy) to provide the required therapy. A therapy regime can become ineffective if the gases are not delivered to the patient with the correct or required characteristics. Often, the most desirable situation is to deliver gases that are fully saturated with water vapour (i.e. at substantially 100% relative humidity) to a user, at a constant flow rate. Other types or variations of therapy regime may call for less than 100% relative humidity. Breathing circuits are not steady-state systems, and it is difficult to ensure the gases are delivered to a user with substantially the correct characteristics. It can be difficult to achieve this result over a range of ambient temperatures, ambient humidity levels, and a range of gas flows at the point of delivery. The temperature, flow rate and humidity of a gases stream are all interdependent characteristics. When one characteristic changes, the others will also change. A number of external variables can affect the gases within a breathing circuit and make it difficult to deliver the gases to the user at substantially the right temperature, flow rate and humidity. As one example, the delivery conduit between the patient or user and the humidifier outlet is exposed to ambient atmospheric conditions, and cooling of the heated, humidified gases within the conduit can occur as the gas travels along the conduit between the exit port of the humidifier chamber and the user interface. This cooling can lead to ‘rain-out’ within the conduit (that is, condensate forming on the inner surface of the conduit). Rain-out is extremely undesirable for reasons that are explained in detail in WO 01/13981.
In order to assist in achieving delivery of the gases stream with the gases having the desired characteristics, prior art systems have used sensors (e.g. temperature and humidity sensors) located at various positions throughout the breathing circuit. Thermistors are generally used as temperature sensors, as these are reliable and inexpensive. Humidity sensors such as the one described in U.S. Pat. No. 6,895,803 are suitable for use with systems that deliver heated humidified gases to a user for therapeutic purposes.
Patent publication WO2001/13981 describes a system for using the output of these sensors to control aspects of the humidified gases supply system. Patent publication WO 2009/145646 another system for using the output of sensors to control aspects of the humidified gases supply system. The content of this publication is hereby incorporated by reference in its entirety.
The conventional approach to providing sensors in the gases stream is to provide a probe that penetrates the tube wall. The probe extends into the gases stream. A thermistor is provided at the probe tip, usually positioned at approximately the middle of the gases stream.
The probe can be fixed in place (for example, where it is provided in a permanent location within the body of the gases supply) or as a removable probe (for example, where it is positioned in part of a replaceable component such as a breathing circuit). In the case of a removable probe, the component to which the probe attaches may include a suitable port with the probe being pushed into the port to protrude into the inside of the conduit.
Positioning the sensor portion of the probe centrally in the gases stream is thought desirable to provide a representative reading of the property of the gases stream (whether this be temperature, humidity or flow). Unfortunately, in this location, the sensor is vulnerable to efforts to clean the inside of the gases passages, for example, with a small sponge on the end of a narrow handle. Furthermore, the projecting sensor can impede the ability to fully clean the gases passage. This can be particularly the case where the protruding probe extends into the passage between an open end of the passage and a bend in the passage. The area between the bend and the probe becomes difficult to access, particularly the surface areas directly behind the probe. Attempts to access these areas can lead to damage to the probe.